Such magnetically operated liquid level indicators are disclosed in U.S. Pat. No. 3,964,312 and G.B. No. 2028505A, and include magnetized indicator elements which are differently colored on opposite faces thereof, and which are housed in a display box disposed in front of the liquid containing vessel. As the magnetized float moves past the magnetized indicator elements, they are rotated in sequence so that those elements which are disposed below the liquid level present one color to the viewer, while those elements which are above the liquid level present the other color. Such indicators have the advantage that there is no mechanical connection between the magnetized float (which may be in a pressurized container) and the magnetized indicator elements (which may be in a display box situated in the atmosphere). This is a particular advantage when handling hazardous fluids since it substantially reduces the possibility of escape of the fluid from the device. However, the magnetized elements of the known indicators (which are made of, for example, pressed steel) only provide a weak magnetic field which is unevenly distributed along their lengths. This is because the magnetic field is distributed such that polarization occurs only at corner regions of the elements and the end regions only of each element behave as a respective pair of parallel weak magnets which have respective like poles disposed at the corner edges, and spaced apart from each other. This tends to produce a rather, weak, unstable magnetic field, and the elements tend to be positionally unstable and readily subject to outside influences such as vibrations or external magnetic fields. Furthermore, there is a possibility that some of the indicator elements may not remain in their set position after the magnetized float has passed them.
Attempts have been made to increase the magnetic field of indicator elements by bonding bar magnets to them, but this provides a rather cumbersome construction.
Both in conventional magnetically operated liquid level indicators, and in indicators embodying the present invention, as later described, the magnetized float may take the form of a bar magnet having a central iron ring to concentrate the magnetic field (see for example GB No. 2028505A). Each end of the bar magnet has a polarity opposite to that of the ring, and it is intended that only the magnetic field of the ring rotates the magnetic indicator elements to the exclusion of the magnetic field of the longitudinal ends of the bar magnet. When using such a float, stops are usually provided for the indicator elements to prevent them from being rotated past their desired position by the magnetic field of the longitudinal ends of the bar magnet.